It is known that in the art of cooling the cooling liquids contained in the radiators of motor vehicles there exists the need to force air onto the radiator itself in order to obtain more rapid dissipation of heat from the liquid to the outside, said forced air flow being obtained by causing rotation of a fan which is normally mounted either on the shaft of the water pump or on a driven shaft carrying a pulley which receives movement from a belt driven by the crankshaft.
It is also known that said fan must be made to rotate only when it reaches a certain predetermined water temperature which is detected by means of a thermostat which actuates an electromagnetic clutch, closing of which causes the fan to start rotating.
Said electromagnetic clutches have the drawback, however, that they start rotation of the fan at the speed of rotation of the actuating pulley, without the possibility of obtaining a variation in the speed of rotation of the fan dependent upon the variation in the speed of operation thereof.
In order to overcome these drawbacks, so-called viscous couplings have also been used, these using rotating discs which are immersed in special greases and are designed to transmit the movement to the fan; said greases are able to vary their state with an increase in the number of revolutions of the disc and are therefore able to impart a speed of rotation of the fan, which, although being dependent upon the speed of rotation of the motive part, is nevertheless different from that of the actuating pulley.
Said couplings, however, have certain drawbacks including those resulting from the fact that control of the temperature is not performed directly on the radiator water, but on a flow of air which is made to pass over an element controlling the circulation of the grease inside the coupling. This indirect measurement of the temperature, in addition to being very imprecise, requires installation of the coupling in the vicinity of the radiator, something which is not always possible for space-related reasons.
In addition to this, the viscous coupling, because of its very nature, has a reaction time which is very slow and is subject to wear of the grease, which tends to become unreliable and at a certain point is no longer able to transmit the movement to the fan with the required accuracy in response, resulting in boiling of the vehicle coolant; said viscous couplings, moreover, are unsuitable for keeping the fan at a standstill when the device is disengaged, as, instead, is required in countries which have a very cold climate.